A common pre-processing step in the digitization of microvolt level signals is amplification with filtering. The signal amplitudes must be increased well in excess of a digitizer's least significant bit (LSB) size so that the quantization noise is negligible when compared to the amplified signal. The required amplification can thus exceed a thousand when working with microvolt level signals. Offsets and 1/f noise in the front-end amplifier are also subject to the same gain, and if they are comparable to the input signal, they can swamp the signal so it is not resolvable or, in the worst case, saturate the amplifier. The presence of thermal noise in the amplifier chain also contributes to the loss of signal fidelity.
Chopper stabilization is a classic technique that is used to lower the offset and 1/f noise in a high gain amplifier. While the chopping circuitry itself is relatively straightforward, i.e. it is comprised of clocked switches before and after the amplifier chain, continuous time low pass filtering and clock delay equalization (to account for the delay through the amplifier chain) must be applied to retrieve the translated signal. Certain implementations also incorporate bandpass filtering into the amplifier chain to reduce the amplifier offset and 1/f noise even further. These circuits are also susceptible to transient noise associated with the switching action of the modulator.
The front-end amplifier offset and 1/f noise may also be reduced by auto-zero techniques. In contrast to the chopper approach, which uses frequency translation, auto-zeroing is a sampling technique where the offset and 1/f noise is stored either on a capacitor or in a digital-analog converter's register and then subtracted from the signal. Auto-zero techniques utilize either a negative feedback loop or a sample-hold to acquire the offset and 1/f noise for eventual correction. The auto-zero approach requires that the offset be periodically sensed and stored through shorting of the amplifier's inputs to a common mode voltage. Thereafter, when the amplifier is operating to amplify the signal, the auto-zero correction is applied. Auto-zero methods have the disadvantage of white noise folding into the signal passband due to the sampling aspect of the technique.
Thus, an alternative approach may be beneficial.